Introduction

Application of mechanical forces to produce a controlled tooth movement forms the basis of orthodontic treatment. According to Newton’s 3rd law of mechanics, for every action there is an equal and opposite reaction. This law works well in orthodontics also. When an orthodontic force is applied for desired tooth movement, there might be undesired unwanted tooth movements also. These unwanted tooth movements if not controlled may comprise the treatment plan and outcome. Hence, the concept of Anchorage is important in orthodontic treatment. Proper anchorage planning is essential during diagnosis, treatment planning, space closure mechanics. Failure to control anchorage can result in undesirable tooth movement, prolonged treatment time, compromised facial aesthetics and occlusion.

Definition

The word anchorage was coined by Alexis Schlange.

According to Graber, Anchorage is nature and degree of resistance to displacement offered by anatomical units when used for the purpose of effective tooth movement. 

According to Proffit, Anchorage is the resistance to unwanted tooth movement. 

For every (desired) action, there is an equal and opposite reaction. 

Anchorage is the resistance to reaction forces that is provided usually by other teeth, occasionally by the palate, sometimes by the head or neck (via extraoral force), and more and more often by anchors screwed into the jaws.

Classification

Based on site of anchorage:

1. Intraoral anchorage:

1. Teeth

2. Palate

3. Alveolar bone

4. Cortical bone

5. Muscular 

2. Extraoral anchorage:

1. Head gear - cervical, high, combination pull.

Based on number of teeth involved:

1. Single/primary - single tooth involved

2. Compound - two or more teeth involved

3. Reinforced - nance palatal button, transpalatal arch, lingual holding arch.

Based on manner:

1. Simple anchorage

2. Stationary - anchor unit resist movement.

3. Reciprocal - anchor unit also moves.

According to Graber:

1. Intramaxillary anchorage

2. Intermaxillary/ Baker’s anchorage

Factors affecting anchorage

Biological factors:

• Size of anchor unit: Increasing the number of teeth in the anchor unit improves the anchorage and minimizes unwanted tooth movement.

• Axial inclination of teeth: Distoaxial inclination has more anchorage than the mesioaxial inclination of posteriors.

• Use of differential force

• Root number: multirooted tooth is more resistant to displacement

• than a single-rooted tooth

• Root form: triangular-shaped root offers greater resistance to

• movement than a conical or ovoid-shaped root

• Root length: longer-rooted tooth is more difficult to move than a shorter-rooted tooth

Mechanical factors:

• Frictional resistance increases the strain on anchorage unit

• Frictional resistance is more with bodily movement and during space closure.

Anchorage value:

The Anchorage value of the tooth is its resistance to movement. It is the function of the root surface area of the tooth.

Simple anchorage:

The manner and application of force tends to displace or change the axial inclination of the tooth or teeth that form the anchorage unit in the plane of space in which the force is being applied.

The resistance of the anchorage unit to tipping is utilized to move another tooth or teeth.

Stationary anchorage:

The manner and application of force tend to displace the anchorage unit bodily in the plane of space in which the force is being applied is termed stationary anchorage.

Example: retraction of maxillary incisors using maxillary molar as anchorage unit.

The advantage that can be obtained by pitting bodily movement of one group of teeth against tipping of another is called stationary anchorage.

Reciprocal anchorage:

The resistance of one or more dental units is utilized to move one or more opposing dental units is reciprocal anchorage.

Example: 

• The reciprocal closure of upper midline diastema

• Correction of Class II malocclusion where intermaxillary elastics are used from the maxillary to the mandibular arch

• Posterior crossbite by the use of ‘through the bite’ elastics

Reinforced anchorage:

Anchorage in which more than one type of resistance unit is utilized is called reinforced anchorage.

Example: palatal removable appliance with clasps over molar bands

Cortical anchorage:

The cortical bone offers more resistance to resorption and if the roots are torqued lingually or buccally, the resistance to movement is increased.

This concept is given by Ricketts.

Example: Space closure in old extraction sites is difficult as the roots encounter cortical bone along the residual ridge.

Muscular anchorage:

Muscles can be used for the anchorage purpose.

Example: lip bumper in case of molar distalization.

According to Nanda:

1. Group A space closure: 25% of posterior anchorage loss and 75% of anterior retraction

2. Group B space closure: more equal amounts of anterior and posterior tooth movement

3. Group C space closure: 75% posterior protraction and 25% of anterior retraction

4. Absolute anchorage: practically 100% of anterior retraction.